fallback model/providers (#259)

Co-authored-by: zuolingxuan <zuolingxuan@bytedance.com>

src/i18n/locales/en/settings.json

@@ -13,6 +13,10 @@
   "aiProviders": {
     "title": "AI Providers",
     "description": "Configure your AI model providers and API keys",
+    "sections": {
+      "model": "Model Settings",
+      "fallback": "Fallback Settings"
+    },
     "add": "Add Provider",
     "custom": "Custom",
     "notRequired": "Not required",
@@ -26,9 +30,19 @@
       "desc": "Configure a new AI model provider",
       "displayName": "Display Name",
       "apiKey": "API Key",
+      "apiKeyConfigured": "An API key is already stored for this provider.",
+      "apiKeyMissing": "No API key is stored for this provider yet.",
       "apiKeyStored": "Your API key is stored locally on your machine.",
+      "replaceApiKey": "Replace API Key",
+      "replaceApiKeyHelp": "Leave this field empty if you want to keep the currently stored API key.",
       "baseUrl": "Base URL",
       "modelId": "Model ID",
+      "fallbackModels": "Fallback Models",
+      "fallbackProviders": "Fallback Providers",
+      "fallbackModelIds": "Fallback Model IDs",
+      "fallbackModelIdsPlaceholder": "gpt-4.1-mini\nanother-model-id",
+      "fallbackModelIdsHelp": "One model ID per line. These models use the current provider config before falling back to other providers.",
+      "noFallbackOptions": "Add another provider first to use it as a fallback target.",
       "cancel": "Cancel",
       "change": "Change provider",
       "add": "Add Provider",
@@ -39,6 +53,9 @@
       "default": "Default",
       "configured": "Configured",
       "noKey": "No API key set",
+      "none": "None",
+      "fallbacks_one": "Fallback: {{names}}",
+      "fallbacks_other": "Fallbacks ({{count}}): {{names}}",
       "setDefault": "Set as default",
       "editKey": "Edit API key",
       "delete": "Delete provider"